L0301P24 - How Tissues Make Organs
__TOC__ Tissues *groups one cells of similar structure and function that service a specific role *cells all have the same embryonic origin *types: **epithelial ***lining, transport, secretion, absorption **connective ***support, strength, elasticity **muscle ***movement **nervous ***information processing, communication and control Organs *a combination of tissues and cells that perform a specific function *e.g.: heart, lung, liver, kidney *composed of four types of tissues **epithelial - lining, transport, secretion, **connective - protection, support, integration (bones ligaments, blood) **muscle - movement **nervous - communication Embryonic Germ Layers How do Tissues Make Organs *tissues make organs by interacting with each other during embryogenesis *cells within tissues send chemical signals to neighbouring tissues to induce: **cell migration (tissue movement) **cell differentiation (tissue changes into another tissue) **replacement (one tissue replaces another) *chemical signals: growth and differentiation **paracrine - local ***WNT and β-catenin signalling ***fibroblast growth factors (FGFs) family ***transforming growth factor (TGF-family **endocrine - hormonal **neuronal signalling Mesenchyme Cells *loosely packed, unspecialized cells from the embryonic mesoderm set in an ECM *give rise to same tissues as mesoderm *can undergo MET (Mesenchyme to Epithelium Transition) Epithelial Cells *one or more layers of polarised cells that line surfaces and sit on a basement membrane *do not move (held together by tight junctions) *can derive from any of the three germ layers *can undergo EMT (Epithelial to Mesenchyme Transition) Organogenesis through Induction *one tissues induces development of another *eg: kidney development, eye development *mechanism: mutual induction between mesenchyme and epithelial cells to cause tubule formation and differentiation Kidney *comprises a system of branched filtration units, filtering blood to remove waste, excess salt and water --> excrete it as urine *participates in water and salt homeostasis Nephron - the Functional Unit *renal corpuscle - blood filtered *proximal, distal convoluted tubules and Loop of Henle - reabsorption of water and ions *collecting ducts - water reabsorption, urine production *ureter - expels urine *mesonephric: during embryonic development *metnephric: adult kidney Kidney Development *whole organ derives from mesoderm **somitic and lateral plate *different tissues within developing organ interact with each other (M and E cells) *involves: **reciprocal induction **epithelial branching **MET **tubule formation (tubulogenesis) Mesoderm *somitic mesoderm (somites) **dermis, cartilage and bone, muscle *intermediate mesoderm **kidneys and gonads *lateral plate mesoderm **circulatory system, extra-embryonic membranes Reciprocal Induction #mesenchyme induces formation of uteric bud from nephric (mesonephric) duct #branching of uteric bud occurs #at tips of branches, uteric bud induces mesenchyme to aggregate and capitate #uteric bud induces mesenchymal cells to undergo MET and fuse with uteric bud #renal tubule extends and bends, differentiating into the convoluted ducts and capsule of the nephron #vascular endothelial cells give rise to glomerulus Induction Secreted Factors *initial signal: mesenchyme to uteric bud **growth factor GDNF (glial-derived neurotrophic factor) **secreted by mesenchyme —> binds to receptor (RET) on nephric duct —> induces growth of uteric bud *reciprocal signal: uteric bud to mesenchyme **fibroblast growth factor- 2 (FGF-2) along with BMP and WNT proteins **then Pax2 and Wt-1 are expressed (both transcription factors) Branching Morphogenesis *in kidney - metanephric mesenchyme induces the uteric bud to grow and bifurcate *i.e. each branch splits into 2 **exponential growth Eye Development *induction: brain (optic vesicle) to lens **optic vesicle grows out from embryonic forebrain induces lens in overlying ectoderm *reciprocal induction: lens to optic vesicle **causes changes in optic vesicle to form optic cup which forms the retina and optic nerve Organogenesis through Replacement *occurs when on tissues replaces another *involves cell differentiation, proliferation and migration Bone *mostly mesodermal **some from neural crest cells - ectodermal Types of Developmental Bone in the Embryo *Intramembranous bone **laid down directly within fibrous connective tissue **most bones of the cranium and face **mesenchymal cells differentiate into osteoblasts **calcification of a matrix secreted by osteoblasts (= ossification) **no cartilage template *Endochondral bone **endo - in; chrondral - cartilage **replaces a cartilage precursor **most non-cranial bones of the skeleton **mesenchymal cells condense into chondrocytes **chondrocytes secrete hyaline cartilage (collagen rich matrix) **specialised regions of the template degenerate, leaving behind the matrix **used by osteoblasts to lay down bone  (= ossification) NOTE: cartilage template does NOT transform into bone; it is replaced by bone tissue Function of the Cartilage Template *a supportive but flexible skeleton for a growing embryo and allows postnatal bone growth *a scaffold for bone deposition *reflects our evolution: primitive vertebrates, and living lampreys had only cartilage skeletons Long Bone Development *bone is mostly mesodermal (some derived from neural crest cells - ectoderm) Endochondral Ossification #in the region where bones are to be formed, cartilage cells commit to the pathway #cartilage cells compact into nodules #cartilage cells proliferate into chondrocytes and lay down a hyaline cartilage matrix within the perichondrium #in the centre, chondrocytes become hypertrophic (rapidly proliferating) and lay down calcium forming a cartilage template #blood vessels enter the calcified cartilage and chondrocytes degenerate leaving a matrix a #osteoblasts invade the matrix and differentiate and secrete their own matrix (osteoid) that will become calcified #extension of ossification along the length of the bone at the epiphyseal growth plates Epiphyseal Growth Plate of a Long Bone Tissue Engineering *growth of new tissues or organs from cells and extracellular matrix scaffold in vitro to produce an organ for implantation back into the donor *therapeutic use in regenerative medicine (skin transplants) & replacing diseased organs *depends on sound knowledge of the normal in vivo development of tissues and organs